Method of forming capacitors containing tantalum

ABSTRACT

The invention pertains to methods of forming capacitors and semiconductor circuit components. In one aspect, the invention includes a method of forming a dielectric layer comprising: a) forming a first tantalum-comprising layer; and b) forming a second tantalum-comprising layer over the first tantalum-comprising layer, the second tantalum-comprising layer comprising nitrogen. In another aspect, the invention includes a method of forming a capacitor comprising: a) forming a first capacitor plate; b) forming a first layer over the first capacitor plate, the first layer comprising tantalum and oxygen; c) annealing the first layer in the presence of an ambient comprising a nitrogen-comprising gas containing at least one compound selected from a group consisting of ammonia, hydrazine and hydrazoic acid; the annealing forming a second layer over the first layer; and d) forming a second capacitor plate over the second layer.

RELATED PATENT DATA

This patent resulted from a divisional application of U.S. patentapplication Ser. No. 08/916,771, which was filed on Aug. 20, 1997, nowU.S. Pat. No. 5,910,880.

TECHNICAL FIELD

The invention pertains to semiconductor circuit components andcapacitors, and to methods of forming capacitors and semiconductorcircuit components.

BACKGROUND OF THE INVENTION

Tantalum pentoxide (Ta₂ O₅) is a desired capacitor dielectric materialdue to its high dielectric constant of about 25. In contrast, othercommonly utilized dielectric materials have much lower dielectricconstants. For instance, silicon nitride has a dielectric constant ofabout 8 and silicon dioxide has a dielectric constant of about 4. Due tothe high dielectric constant of Ta₂ O₅, a thinner layer of Ta₂ O₅ can beutilized in capacitor constructions to achieve the same capacitance asthicker layers of other materials.

Semiconductive capacitors comprise a first conductive plate and a secondconductive plate, with a dielectric layer formed between the plates.Commonly, the conductive plates comprise doped polysilicon, with one orboth of the plates comprising a rugged form of polysilicon, such as, forexample, hemispherical grain polysilicon.

It is highly desired to utilize Ta₂ O₅ as the dielectric layer due tothe dielectric properties discussed above. Unfortunately, the chemicalvapor deposition (CVD) processes by which Ta₂ O₅ is formed adverselycomplicate its incorporation into semiconductive capacitors. Forinstance, Ta₂ O₅ is not typically deposited onto a first polysiliconplate, nor is a second polysilicon plate-typically directly depositedonto Ta₂ O₅. The CVD processes by which Ta₂ O₅ is formed adverselyaffect underlying and overlying polysilicon layers unless suchpolysilicon layers are first protected with barrier layers.Specifically, Ta₂ O₅ is typically formed by a CVD process in whichTa(OC₂ H₅)₅ and oxygen are combined. Unless a polysilicon plate isprotected by a barrier layer before such CVD deposition over thepolysilicon, the oxygen of the CVD process will react with thepolysilicon to disadvantageously form a layer of silicon dioxide overthe polysilicon. Present methods for protecting the polysilicon includeprovision of a silicon nitride layer over the polysilicon prior toformation of Ta₂ O₅. The silicon nitride layer is typically 10 to 20angstroms thick. Also, unless a Ta₂ O₅ layer is first covered with abarrier layer before formation of polysilicon over the Ta₂ O₅ layer, thepolysilicon will react with oxygen in the Ta₂ O₅ layer todisadvantageously form silicon dioxide.

An example prior art process for forming a capacitor 10 having a Ta₂ O₅dielectric layer is illustrated in FIG. 1. A polysilicon first capacitorplate 12 is formed over a substrate 14. A silicon nitride layer 16 isformed over polysilicon layer 12. A Ta₂ O₅ dielectric layer 18 is formedover silicon nitride layer 16 by the above-described CVD process. Afterthe CVD of Ta₂ O₅ layer 18, the layer is typically subjected to ananneal in the presence of an oxygen ambient. The anneal drives anycarbon present in layer 18 out of the layer and advantageously injectsadditional oxygen into layer 18 such that the layer uniformly approachesa stoichiometry of five oxygen atoms for every two tantalum atoms. Theoxygen anneal is commonly conducted at a temperature of from about 400°C. to about 1000° C. utilizing an ambient comprising an oxygencontaining gas. The oxygen containing gas commonly comprises one or moreof O₃, N₂ O and O₂. The oxygen containing gas is typically flowedthrough a reactor at a rate of from about 0.5 slm to about 10 slm.

Ta₂ O₅ layer 18 is typically from about 40 angstroms to about 150angstroms thick and can be either amorphous or crystalline. It is notedthat Ta₂ O₅ is generally amorphous if formed below 600° C. and will becrystalline if formed, or later processed, at or above 600° C.Typically, a Ta₂ O₅ layer is deposited as an amorphous layer and theabove-described oxygen anneal is conducted at a temperature of 600° C.or greater to convert the amorphous Ta₂ O₅ layer to a crystalline layer.

A second nitride layer 20 is deposited over Ta₂ O₅ layer 18. Secondnitride layer 20 typically comprises TiN or WN. A second capacitor plate22 is formed over nitride layer 20. Second capacitor plate 22, likefirst capacitor - plate 12, typically comprises doped polysilicon ordoped rugged polysilicon. It is noted that the top electrode of the Ta₂O₅ capacitor can comprise only TiN or WN layer 20, or can comprise thelayer 20 and layer 22 stack.

The formation of layer 20 is typically done by a chemical vapordeposition process, as opposed to a sputtering type process, to achieveacceptable conformity in high aspect ratio capacitor devices. Such CVDprocesses use either metal organic precursors or organometallicprecursors. Either precursor contains carbon and results in thedeposition of a barrier layer 20 which typically includes large amountsof carbon, commonly from about 10 to about 15 volume percent, andsometimes as much as 30 volume percent. Although such carbon typicallydoes not adversely impact the function or conductivity of the nitridelayer 20, subsequent wafer processing can cause carbon from layer 20 todiffuse into Ta₂ O₅ layer 18. Carbon diffusing into Ta₂ O₅ layer 18 candisadvantageously cause layer 18 to leak current, and in extreme casescan convert an intended capacitor device 10 into a device that behavesmore like a resistor than a capacitor.

An additional disadvantage that can occur during placement of a nitridebarrier layer 20 over Ta₂ O₅ layer 18 is that there is typically someformation of the undesired compound TiO₂ at an interface between Ta₂ O₅layer 18 and barrier layer 20.

It would be desirable to develop alternative methods of utilizing Ta₂ O₅in integrated circuit construction.

SUMMARY OF THE INVENTION

In one aspect, the invention encompasses methods of forming a dielectriclayer. A first tantalum-comprising layer is formed and a secondtantalum-comprising layer is formed over the first tantalum-comprisinglayer. The second tantalum-comprising layer comprises nitrogen.

In another aspect, the invention encompasses a method of forming acapacitor. A first capacitor plate is formed and a first layer is formedover the first capacitor plate. The first layer comprises tantalumoxide. A second layer is formed over the first layer. The second layercomprises tantalum and nitrogen. A second capacitor plate is formed overthe second layer.

In another aspect, the invention encompasses a method of forming acapacitor. A first capacitor plate is formed. A first layer is formedover the first capacitor plate. The first layer comprises tantalum andoxygen, and is substantially void of carbon. A barrier layer is formedover the first layer. A metal nitride layer is formed over the barrierlayer. A second capacitor plate is formed over the metal nitride layer.The metal nitride layer is processed at a sufficient temperature todiffuse carbon from the metal nitride layer. The barrier layersubstantially prevents the diffused carbon from permeating into thefirst layer.

In another aspect, the invention encompasses a semiconductor circuitcomponent comprising a first tantalum-comprising layer and a secondtantalum-comprising layer over the first tantalum-comprising layer. Thesecond tantalum-comprising layer comprises nitrogen.

In another aspect, the invention encompasses a capacitor. The capacitorcomprises a first capacitor plate and a first layer over the firstcapacitor plate. The first layer comprises tantalum and oxygen. Thecapacitor further includes a second layer over the first layer. Thesecond layer comprises tantalum and nitrogen. Additionally, thecapacitor includes a second capacitor plate over the second layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIGS. 1 is a fragmentary diagrammatic cross-sectional view of asemiconductor wafer fragment illustrating a prior art capacitorconstruction.

FIG. 2 is a fragmentary diagrammatic cross-sectional view of asemiconductor wafer fragment at a preliminary processing step of thepresent invention.

FIG. 3 is a fragmentary cross-sectional view of the FIG. 2 waferfragment shown at a processing step subsequent to that of FIG. 2.

FIG. 4 is a fragmentary cross-sectional view of the FIG. 2 waferfragment shown at a processing step subsequent to that of FIG. 3, inaccordance with a first embodiment of the present invention.

FIG. 5 is a fragmentary cross-sectional view of the FIG. 2 waferfragment shown at a processing step subsequent to that of FIG. 3, inaccordance with a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

Semiconductor processing methods of the present invention are describedwith reference to FIGS. 2-5. Referring to FIG. 2, a semiconductor waferfragment 30 is illustrated at a preliminary processing step inaccordance with the present invention. Wafer fragment 30 comprises asubstrate 32 upon which is formed a first capacitor plate 34. Firstcapacitor plate 34 preferably comprises conductively doped polysilicon,and most preferably comprises conductively doped rugged polysilicon,such as, for example, hemispherical grain polysilicon. First capacitorplate 34 can be formed by conventional methods.

A nitride layer 36 is formed over first capacitor plate 34 and atantalum-comprising layer 38 is formed over nitride layer 36.Tantalum-comprising layer 38 preferably comprises Ta₂ O₅, and nitridelayer 36 can comprise, for example, silicon nitride. Layers 34, 36 and38 can be formed by conventional methods, such as, for example, themethods discussed above in the "Background Of The Invention" section ofthis disclosure. In the context of the present invention, an additionaltantalum-comprising layer will be formed over layer 38. Accordingly,layer 38 may be referred to as a first tantalum-comprising layer.

Referring to FIG. 3, a second tantalum-comprising layer 40 is formedover first tantalum-comprising layer 38. Second tantalum-comprisinglayer 40 is a barrier layer preferably comprising tantalum and nitrogen,such as in the form Ta₂ N. Second tantalum-comprising layer 40 canadditionally comprise oxygen, and may, for example, comprise a compoundhaving the general formula TA_(x) O_(y) N_(z).

Second tantalum-comprising layer 40 can be formed by depositing a layerof Ta₂ N, or Ta_(x) O_(y) N_(z), over first tantalum-comprising layer38. Alternatively, and more preferred, second tantalum-comprising layer40 is formed by exposing an outer surface of first tantalum-comprisinglayer 38 to a nitrogen-comprising ambient. Such nitrogen-comprisingambient can, for example, comprise one or more gases selected from agroup consisting of hydrazine, hydrazoic acid, ammonia and NF₃.Alternatively, the nitrogen-comprising ambient can consist essentiallyof hydrazine, can consist of essentially of hydrazoic acid, can consistessentially of ammonia, or can consist essentially of NF₃. Thenitrogen-comprising gas preferably does not consist essentially of N₂.The exposure to the nitrogen-comprising ambient can utilize a plasma(rf, ECR or remote plasma) to generate an active nitrogen species. Theplasma can contain the nitrogen-comprising ambient, and a diluent suchas, N₂, H₂, Ar, and/or He.

The above-described exposure of an outer surface of layer 38 to anitrogen-comprising ambient preferably comprises an anneal of layer 38at a temperature of from about 350° C. to about 900° C. The annealtypically comprises heating at least the exposed outer surface of layer38 to such temperature. If the nitrogen-comprising ambient consistsessentially of hydrazine, the annealing temperature. is preferably fromabout 350° C. to about 600° C. If the nitrogen-comprising ambientconsists essentially of ammonia, the annealing temperature is preferablyfrom about 600° C. to about 900° C. It is noted that, as discussedabove, Ta₂ O₅ transforms from an amorphous material to a crystallinematerial at about 600° C. Accordingly, it may be desired to utilize ahydrazine-comprising gas for forming second tantalum-comprising layer 40when it is desired to keep Ta₂ O₅ layer 38 in an amorphous form. Incontrast, it may be desired to utilize an ammonia-comprising gas when itis desired to convert an amorphous Ta₂ O₅ layer 38 to a crystallineform. Methods for exposing a wafer surface to hydrazine, ammonia, NF₃and/or hydrazoic acid in a semiconductor processing reactor at the isabove-discussed temperatures are known to persons of ordinary skill inthe art.

First tantalum-comprising layer 38 is preferably exposed to thenitrogen-comprising ambient for a time of from about 30 seconds to about10 minutes. It is noted that the formation of second tantalum-comprisinglayer 40 from the interaction of the nitrogen-comprising ambient withfirst layer 38 will be a self-limiting process which will generallyterminate by about 10 minutes. Second tantalum-comprising layer 40 willgenerally be less than or equal to about 20 angstroms thick at thetermination of such process.

FIGS. 4 and 5 illustrate alternative processing methods which can beutilized after formation of second tantalum-comprising layer 40.Referring to the first embodiment processing method of FIG. 4, a metalnitride layer 42 is formed over second tantalum-comprising layer 40.Metal nitride layer 42 can comprise materials known to persons ofordinary skill in the art, such as, for example, TiN or WN, and can beformed by conventional methods, such as, for example, CVD.Advantageously, if metal nitride layer 42 is formed by a CVD process,carbon present in such layer due to the CVD process will be preventedfrom diffusing into a Ta₂ O₅ layer 38 by the barrier layer 40.Accordingly in contrast to prior art processes, Ta₂ O₅ layer 38 willremain substantially void of carbon in spite of the provision of metalnitride layer 42 overlying Ta₂ O₅ layer 38. A second capacitor plate 44is formed over metal nitride layer 42. Second capacitor plate 44 cancomprise, for example, conductively doped polysilicon. Second capacitorplate 44 can be formed by conventional processes.

Referring to FIG. 5, a second embodiment process for completing acapacitor structure of the present invention is illustrated. In thisembodiment, a second capacitor plate 50 is formed directly over secondtantalum-comprising layer 40. Second capacitor plate 50 can comprise,for example, conductively doped polysilicon. Second tantalum-comprisinglayer 40 functions as a barrier layer between the polysilicon of layer50 and the Ta₂ O₅ of layer 38 to prevent an undesired formation ofsilicon dioxide between layers 50 and 38. Accordingly, barrier layer 40permits polysilicon layer 50 to be directly deposited over atantalum-comprising layer, in contrast to prior art processes whereinthe polysilicon layer was formed over a non-tantalum-comprising metalnitride layer. It is noted that tantalum-comprising barrier layer 40 notonly prevents diffusion of carbon from a metal nitride layer 42 into theTa₂ O₅ of layer 38, but also substantially prevents diffusion of oxygenfrom layer 38 into layers above tantalum-comprising layer 40. It is alsonoted that barrier layer 40 differs form the prior art barrier layer 20(described above with reference to FIG. 1) in that barrier layer 40 issubstantially void of carbon, i.e., comprises less than 10 volumepercent of carbon and preferably less than about 5 volume percent.

The processing described above with reference to FIGS. 2-5 formssemiconductor circuit components comprising first tantalum-comprisinglayer 38 and an overlying second tantalum-comprising layer 40. Althoughthe above described processing is directed toward capacitorconstructions, it is to be understood that such semiconductor circuitcomponents can have application to other electrical structures, besidescapacitors, in which a high-dielectric material is desired.

To aid in interpretation of the claims that follow, the term"semiconductive substrate" is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term "substrate" refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A method of forming a capacitorcomprising:forming a first capacitor plate; forming a first layer as adielectric over the first capacitor plate, the first layer comprisingtantalum and having an exposed outer surface; forming a second layerover the first layer by exposing the outer surface of the first layer toa nitrogen-comprising gas comprising hydrazoic acid, NF₃, or mixturesthereof, the second layer comprising tantalum and nitrogen; and forminga second capacitor plate over the second layer.
 2. The method of claim 1wherein the nitrogen-comprising gas consists essentially of hydrazoicacid.
 3. The method of claim 1 wherein the nitrogen-comprising gasconsists essentially of NF₃.
 4. The method of claim 1 wherein the firstlayer comprises an outer surface and wherein the forming of the secondlayer comprises chemical vapor depositing a material comprising tantalumand nitrogen onto the outer surface.
 5. A method of forming a capacitorcomprising:forming a first capacitor plate comprising polysilicon;forming a silicon nitride layer over the polysilicon; forming a firstlayer as a dielectric over the silicon nitride layer, the first layercomprising tantalum; forming a second layer over the first layer, thesecond layer comprising at least some of the tantalum of the first layerand nitrogen; and forming a second capacitor plate over the secondlayer.
 6. A method of forming a capacitor comprising:forming a firstcapacitor plate; forming a first layer as a dielectric over the firstcapacitor plate, the first layer comprising tantalum and having anexposed outer surface; forming a second layer over the first layer byexposing the outer surface of the first layer to a nitrogen-comprisinggas, the second layer comprising some of the tantalum of the first layerand some of the nitrogen of the nitrogen-comprising gas; and forming asecond capacitor plate over the second layer.
 7. The method of claim 6wherein the nitrogen-comprising gas comprises one or more gases selectedfrom a group consisting of hydrazine, hydrazoic acid, NF₃ and ammonia.8. The method of claim 6 wherein the nitrogen-comprising gas consistsessentially of hydrazine.
 9. The method of claim 6 wherein thenitrogen-comprising gas consists essentially of hydrazoic acid.
 10. Themethod of claim 6 wherein the nitrogen-comprising gas consistsessentially of NF₃.
 11. The method of claim 6 wherein thenitrogen-comprising gas consists essentially of ammonia.
 12. The methodof claim 6 wherein the exposure to the nitrogen-comprising gas comprisesutilizing a plasma to generate an active nitrogen species.
 13. Themethod of claim 6 wherein the exposing the outer surface of the firstlayer occurs at a temperature of from about 350° C. to about 900° C. 14.The method of claim 13 wherein the nitrogen-comprising gas comprises oneor more gases selected from a group consisting of hydrazine and ammonia.15. The method of claim 14 wherein the nitrogen-comprising gas consistsessentially of hydrazine and wherein the temperature is from about 350°C. to about 600° C.
 16. The method of claim 14 wherein thenitrogen-comprising gas consists essentially of ammonia and wherein thetemperature is from about 600° C. to about 900° C.
 17. The method ofclaim 6 wherein the second layer comprises tantalum nitride.
 18. Themethod of claim 6 wherein the second layer comprises tantalum, oxygenand nitrogen.
 19. The method of claim 6 further comprising forming ametal nitride between the second layer and the second capacitor plate.20. The method of claim 19 wherein the second capacitor plate comprisesTiN and is formed from one or more organic precursors comprising Ti andN.
 21. The method of claim 19 wherein the second capacitor platecomprises WN and is formed from one or more organic precursorscomprising W and N.
 22. The method of claim 6 wherein the secondcapacitor plate comprises polysilicon and further comprising forming ametal nitride between the second layer and the second capacitor plate.23. The method of claim 6 wherein the second capacitor plate comprisespolysilicon.
 24. The method of claim 6 wherein the first layer comprisesoxygen.
 25. The method of claim 6 wherein the first layer consistsessentially of Ta₂ O₅.
 26. The method of claim 6 wherein the forming thesecond layer over the first layer is self-limiting.
 27. A method offorming a capacitor comprising:forming a first capacitor plate; forminga first layer as a dielectric over the first capacitor plate, the firstlayer comprising tantalum and having an exposed outer surface; forming asecond layer over the first layer by exposing the outer surface of thefirst layer to a nitrogen-comprising gas at a temperature of from about350° C. to about 900° C., and for a time of from about 30 seconds toabout 10 minutes, the second layer comprising tantalum and nitrogen; andforming a second capacitor plate over the second layer.
 28. A method offorming a capacitor comprising:forming a first capacitor plate; forminga tantalum-comprising layer as a dielectric over the first capacitorplate, the tantalum-comprising layer comprising oxygen and beingsubstantially void of carbon; forming a barrier layer over thetantalum-comprising layer; forming a metal layer comprising carbon overthe barrier layer; forming a second capacitor plate over the metallayer; and processing the metal layer at a sufficient temperature todiffuse carbon from the metal layer, the barrier layer substantiallypreventing such carbon from diffusing into the tantalum-comprisinglayer.
 29. The method of claim 28 wherein the metal layer comprisesmetal nitride.
 30. The method of claim 29 wherein the metal nitridelayer comprises TiN or WN and is formed by chemical vapor deposition.31. The method of claim 28 wherein the barrier layer comprises tantalumand nitrogen.
 32. The method of claim 31 wherein the barrier layercomprises tantalum nitride.
 33. The method of claim 31 wherein thebarrier layer comprises tantalum, oxygen and nitrogen.
 34. A method offorming a capacitor comprising:forming a first capacitor plate; forminga tantalum-comprising layer as a dielectric over the first capacitorplate, the tantalum-comprising layer comprising oxygen; forming abarrier layer over the tantalum-comprising layer, the barrier layercomprising less than 10 volume percent of carbon and comprising some ofthe tantalum of the tantalum-comprising layer; and forming a secondcapacitor plate over the barrier layer.
 35. The method of claim 34wherein the barrier layer comprises tantalum and nitrogen.
 36. A methodof forming a capacitor comprising:forming a first capacitor plate;forming a first layer as a dielectric over the first capacitor plate,the first layer comprising tantalum and oxygen; annealing the exposedfirst layer in the presence of an ambient comprising anitrogen-comprising gas containing at least one compound selected from agroup consisting of ammonia, hydrazine, NF₃ and hydrazoic acid; theannealing forming a second layer over the first layer comprising some ofthe tantalum of the first layer; and forming a second capacitor plateover the second layer.
 37. The method of claim 36 wherein thenitrogen-comprising gas is within a plasma during the annealing.
 38. Themethod of claim 36 wherein the nitrogen-comprising gas consistsessentially of hydrazine and wherein the annealing comprises atemperature of from about 350° C. to about 600° C.
 39. The method ofclaim 36 wherein the nitrogen-comprising gas consists essentially ofammonia and wherein the annealing comprises a temperature of from about600° C. to about 900° C.
 40. A method of forming a capacitorcomprising:forming a first capacitor plate; forming a first layer as adielectric over the first capacitor plate, the first layer comprisingtantalum and having an exposed outer surface; forming a second layerover the first layer by exposing the outer surface of the first layer toa nitrogen-comprising gas consisting essentially of hydrazine at atemperature of from about 350° C. to about 600° C., the second layercomprising tantalum and nitrogen; and forming a second capacitor plateover the second layer.
 41. A method of forming a capacitorcomprising:forming a first capacitor plate; forming a first layer as adielectric over the first capacitor plate, the first layer comprisingtantalum and having an exposed outer surface; forming a second layerover the first layer by exposing the outer surface of the first layer toa nitrogen-comprising gas consisting essentially of ammonia at atemperature of from about 600° C. to about 900° C., the second layercomprising tantalum and nitrogen; and forming a second capacitor plateover the second layer.
 42. A method of forming a capacitorcomprising:forming a first capacitor plate; forming a first layer as adielectric over the first capacitor plate, the first layer comprisingtantalum and oxygen; annealing the exposed first layer at a temperatureof from about 350° C. to about 600° C. in the presence of an ambientcomprising a nitrogen-comprising gas consisting essentially ofhydrazine, the annealing forming a second layer over the first layer;and forming a second capacitor plate over the second layer.
 43. A methodof forming a capacitor comprising:forming a first capacitor plate;forming a first layer as a dielectric over the first capacitor plate,the first layer comprising tantalum and oxygen; annealing the exposedfirst layer at a temperature of from about 600° C. to about 900° C. inthe presence of an ambient comprising a nitrogen-comprising gasconsisting essentially of ammonia, the annealing forming a second layerover the first layer; and forming a second capacitor plate over thesecond layer.